Hermetic seal for optical waveguide ribbon feed through

ABSTRACT

A technique for hermetically sealing an optical component in a metal package is described. Variations of the technique are described in which optical communication between the optical component and the outside environment is achieved with a ribbon fibre.

FIELD OF THE INVENTION

[0001] The field of the invention relates generally to hermetic sealingof optical and optoelectronic components and more specifically to thehermetic sealing of a set of optical waveguides such as a ribbon offibres.

BACKGROUND OF THE INVENTION

[0002] The expansion of digital data traffic resulting from increaseduse of the Internet has led to the deployment of optical networks. Thesenetworks transmit large quantities of information very quickly howeverthey rely on expensive optical components. In order to ensure the properfunctioning of the optical components it is often necessary to isolatethem from their environment. Isolating the optical component orcomponents from moisture is very beneficial because water acceleratesaging and corrosion of the optical component. Additionally, water isknown to have highly detrimental effects on adhesives, which isunfortunate because adhesives are a convenient way of attaching opticalcomponents together, particularly through a light-transmitting surface.As one of skill in the art of hermetic sealing of optical componentswill be aware, it is important that the hermetic seal be sufficientlyrobust that the seal is not compromised during the working life of theproduct. When products are sold with twenty-year guarantees, thehermetic seal of each subcomponent is expected to last the twenty years.Generally, it is not common to test each individual component forhermetic integrity. Instead, it is common practice to establish a methodof hermetic sealing that is highly reliable and use spot checks toensure that the method is working effectively. Verifying the integrityof a hermetic seal requires time and uses very costly, specializedequipment. For this reason, it is beneficial to use methods of hermeticsealing which are well established and proven in productionenvironments.

[0003] Preventing moisture from entering a package containing an opticalcomponent is a difficult task because water molecules will penetratemicroscopic cracks. A conventional optical component package is a metalbox with one or more features for allowing glass waveguides to penetratethe exterior of the box. Unfortunately, it is hard to seal a metal boxabout a glass waveguide without leaving a crack. The prior art of KovatsU.S. Pat. No. 4,413,881, herein referred to as Kovats, teaches that aglass fibre inserted in a metal tube may be hermetically sealed to thetube by injecting a melted solderable alloy, such as BiSn in the tube,thereby plugging the tube. If the alloy expands during solidificationthen it will squeeze the optical fibre and help to prevent the formationof cracks between the optical fibre and the solder. It should be notedthat optical fibres are generally glass and that most molten metals willnot ordinarily wet to a ceramic surface. The Kovats prior art avoidsthis problem by filling a tube with molten solder while the opticalfibre is in the tube. Using this method, pressure between the tube andthe fibres caused by the solidification of the solder forms the hermeticseal.

[0004] While the Kovats prior art teaches a method of sealing a singlefibre hermetically, it does not provide a simple means of sealing amulti-fibre array for example, a ribbon fibre, hermetically. If a ribbonof optical fibres is separated and each individual optical fibre issealed independently then a variety of problems result. For example, themetal tubes used in forming the hermetic seal are substantially largerthan the fibre. Typically these tubes are spaced approximately 0.20″ (5mm) between fibre centers. This is not a significant concern for largepackages with few fibres however it is not uncommon to produce arrayedwaveguide gratings (AWG) with over forty optical fibres which ideallyexit the package through the same face and the spacing is typically 250microns between adjacent fibre centers. Indeed as AWGs become moresophisticated there will be a need for even larger numbers of fibresprovided at one face of an optical device to be hermetically sealed.

[0005] Alternatively, if a ribbon of fibres were sealed in a packagewall according to the method of Kovats, the ribbon would likely be proneto twisting as the solder flows around it. This likely induces stressand thereby reduces the optical performance of the packaged opticalcomponent. Additionally, great care must be taken to ensure that thesolder flows around all of the fibre as the fibre themselves hinder theflow of solder. Thus, when sealing a ribbon of fibre it is far morelikely that voids and cracks will be present in the seal after cooling,thus compromising the hermetic performance of the seal.

[0006] Alternative methods of sealing an optical fibre are well known inthe art. For example, in order to ensure that the solder wets thesurface of the optical fibre it is known to metalize the fibre prior toencasing them in solder. Since the glass fibre is now encased in metalprior to being immersed in molten solder the solder easily bonds to themetalized surface of the fibre. At first glance, this solution appearshighly advantageous however it has disadvantages. For example,metalizing the fibre is a slow process that typically involves a vacuumdeposition machine. These machines are expensive and it is highlyrecommended that a skilled operator oversee them. Although many fibrescan be metalized in one use of the machine, the process for metalizingfibres is very slow as only tiny amounts of metal are deposited at agiven time. Consequently, if this process were to be incorporated foruse with a flexible manufacturing environment then a JIT (just-in-time)manufacturing schedule would be very difficult to incorporate.Additionally, metalized optical fibres are very fragile and easilydamaged in handling. Any separation of the metalized layer from theoptical fibre will likely compromise the effectiveness of a hermeticseal. Alternatively, metalized optical ribbon fibre with up to eightindividual waveguides are available commercially however they are verycostly and their fragile nature makes shipping them very costly as well.However, large number count metalized ribbon fibre is still notavailable, because it is difficult to metalize so many fibre uniformlyat a time. Typically, metalized ribbon fibre featuring over twelveindividual waveguides is not commercially available.

[0007] It would be beneficial to provide a simple, effective method ofhermetically sealing optical components that supports the sealing of afibre array or a ribbon of fibres without inducing stress on theindividual fibre. Preferably, such a method is for allowing the fibrearray to be sealed hermetically without requiring costly and complexequipment. Additionally, it would be beneficial if such a methodincorporated proven hermetic sealing technology and not involvepreparing components in advance and storing them for later use.

SUMMARY OF INVENTION

[0008] The invention teaches the design of a junction for forming ahermetic seal about an optical waveguide, said junction comprising: atube for providing fluid communication between a first orifice and asecond orifice, a cap disposed for covering the first orifice, said capfor reducing a flow of molten metal through the first orifice when theoptical waveguide is disposed within the cap, said tube beingsufficiently wide to support the optical waveguide disposed between thefirst orifice and the second orifice while simultaneously permittingmolten metal at a predetermined temperature to flow within the tube,such that, in use, the optical waveguide is disposed within the tube,molten metal is provided within the tube and flows to the cap and themolten metal solidifies, thereby forming a hermetic seal between theoptical waveguide and the tube.

[0009] Further the invention describes a method of forming an opticalwaveguide hermetic seal comprising the steps of: disposing an opticalwaveguide through a cap; abutting the cap to a first orifice of a tube;providing molten metal to the tube such that the molten metal providedin a channel of the tube flows toward the first orifice of the tube;and, allowing the molten metal to solidify, thereby forming a hermeticseal between the tube and the optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will now be described with reference to thedrawings in which

[0011]FIG. 1 is a drawing of a prior art hermetic joint;

[0012]FIG. 2 is a section view of a joint with a ribbon of opticalfibres in which a void has compromised the seal;

[0013]FIG. 3 is a section view of an embodiment of the invention priorto providing solder; and,

[0014]FIG. 3a is a section view of an embodiment of the invention withmolten solder surrounding the ribbon of optical fibres.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The invention provides a method and apparatus for producing arobust hermetic seal about a ribbon of optical fibres. Referring to FIG.1, a prior art diagram of hermetic seal produced by the hermetic sealingtechnique of Kovats is illustrated in a section view shown in FIG. 1.The drawing shows an optical fibre encased in solder. The fibre 1 andsolder are contained within a metal tube 3 that helps to keep the solderin compression. The solder in provided to the tube via an orifice 2.

[0016] This technique is poorly suited to hermetically sealing a fibrearray or ribbon for a variety of reasons. For example, in order toproduce a robust hermetic seal, the optical fibre is surrounded bysolder. Since glass optical fibres do not wet with most molten soldersit is critical that the solder be proximate the surface of the fibreswhen it solidifies. FIG. 2 illustrates a ribbon of optical fibres 21encased in solder 22. Similarly to the prior art, the sheath material 27used to cover and bind together the individual fibre of the ribbon ofoptical fibres 21 has been stripped leaving an exposed region 24.Typically the sheath material is not sufficiently well bonded to thefibres to form a hermetic seal and often the sheath material is aplastic that is not hermetic. Since the ribbon of fibres is a set offibres in close proximity, it is difficult to get the solder 22 into thecorrect position necessary to uniformly coat the exposed region 24 ofthe fibres 21. This increases the likelihood of there being voidsbetween the optical fibres. Such a void 23 is shown proximate thefibres. This void 23 is expected to cause the adjacent fibres 25 and 26to be pushed towards the void when the solder 22 expands. Thedisplacement of the fibres 25 and 26 induces stress on the fibers.Additionally, in an extreme case, the fibers will kink somewhat andwhich leads to changes in their polarization characteristics.Additionally, it is unlikely that a void is entirely eliminated when thesolder expands and therefore, the resulting joint will likely not behermetic.

[0017] Thus, in order to produce a consistent and robust hermetic sealit is important to use a method of providing molten metal that reducesthe likelihood of voids forming. Ideally, each of the optical fibresfrom the ribbon fibre is surrounded by molten metal in order to ensurethat the fibres are not stressed. However, separating each of the fibresof the ribbon and sealing them independently is not practical.

[0018] Referring to FIG. 3, a section view of a package wall with ahermetic junction suitable for accommodating ribbon of optical fibres isshown. The package 30 includes a hollow tube section 31. The tubesection 31 will support the ribbon fibre and the molten solder. A cap 32has been attached to the exterior of the tube section 31. The cap 32 hasa feature for allowing a ribbon fibre 33 to penetrate the cap. Theribbon fibre 33 has been stripped of its plastic sheath in a region 33 acontained within the tube section 31. The ribbon fibre 33 is held at oneend by the cap 32 and at the other end by a jig 36. The jig 36 and thecap 32 ensure that the ribbon fibre 33 is under a slight amount oftension in the tube section 31. The cap 32 is not hermetic. The opening34 of the tube section 3 l opposite to the cap 32 remains accessible tospecialized tools despite the presence of the jig 36.

[0019] Referring to FIG. 3a, molten metal 35 is provided to the tubesection 31 from the opening 34. The molten metal 35 flows within thetube section 31 and covers the exposed region 33 a. The orientation ofthe tube section 31 with the cap 32 down helps to bias any air pocketsup and out of the molten metal 35. The cap 32 reduces the amount ofmolten metal draining out the bottom of the tube section. Since theribbon fibre 33 is held under tension between the jig 36 and the cap 32the ribbon fibre 33 resists being pushed against the side of the tubesection 31, thus ensuring that the ribbon fibre 33 is not displaced whenthe molten solder expands during solidification.

[0020] Additionally, the use of the said design permits the use ofsolders absent flux. As one of skill in the art will be aware, the useof acidic flux is cause for concern because even a mild acid will have adetrimental effect on a hermetic joint. Additionally, many opticalcomponents, such as InP based integrated circuits and MEMS (MicroElectro Mechanical System) devices, are very sensitive to contaminationfrom flux therefore eliminating flux eliminates failure due to fluxcontamination during sealing.

[0021] Clearly, the diameter of the tube section 31 is chosen to besufficiently large for accommodating the ribbon fibre 33 as well as theflowing solder 35. Providing sufficient space for the solder to flowwithin the tube section and around the ribbon fibre 33 helps to providea consistent process. If the solder 35 should push the ribbon fibre 33to one side of the tube section 31 the slight tension provided to theribbon fibre 33 from the cap 32 and jig 36 cause the ribbon fibre toreturn to an area proximate the center of the tube section 31 and awayfrom the walls. This helps to prevent the formation of voids between anyof the individual fibres and the tube section 31. Thus, the solder isable to wet a complete diameter of the tube section 31 and the fibres ofthe ribbon fibre 33 remain substantially straight relative to the tubesection 31. The molten solder 35 expands slightly on solidification andthus it exerts pressure on the package and the optical fibres. Thispressure helps to ensure that no cracks form.

[0022] Once the molten solder has solidified and cooled, the cap 32 isoptionally removed to visually inspect the hermetic joint.Alternatively, the cap 32 is made from a flexible material capable ofresisting high temperature failure and is used as a strain relief forthe fibres exiting the tube section. Since most hermetically sealedoptical fibres require a strain relief the addition of the cap does notsubstantially increase costs of manufacturing. Although, this embodimentclearly incorporates a ribbon of optical fibres, an alternativeembodiment of the invention hermetically seals a single optical fibre.Clearly, the cross section of the ribbon fibre or single fibredetermines the appropriate tube shape. While this simplifies the processand helps to ensure that a good hermetic seal results, alternativeembodiments exist. For example, if the material of the tube section hasa higher coefficient of thermal expansion than the solidified solder itis apparent to one of skill in the art of mechanical design that thetube section squeezes the solder as it cools thereby enhancing the sealit creates. Provided the correct amount of cooling and the correctthermal expansion characteristics of the various components, a robusthermetic seal is produced.

[0023] In an alternative embodiment of the invention, the package isprovided to allow the ribbon of optical fibres to be held horizontally.In this embodiment, separate caps are used at each end of the tubesection and the molten solder is provided from a small orifice in fluidcommunication with the tube section.

[0024] In yet another embodiment, the material for the cap is not heldfirmly to the package but merely held in proximity to the package by asecond jig. When the solder has solidified, the second jig is removed.Alternatively, no cap is incorporated and the second jig acts as a cap.In this case the second jig has a plugging region for reducing the flowof solder out the end of the tube section. This plugging region has asurface that [will] substantially prevents bonding to the solder.

[0025] In yet another alternative embodiment of the invention, the tubesection is designed to accommodate more than one ribbon of opticalfibres. This will allow an even larger number of optical waveguides topenetrate the package. However, it is recommended that the cap and tubegeometry be redesigned for different combinations of ribbon fibre inorder to produce a hermetic joint according to the invention.

[0026] It will be apparent to one of skill in the art of mechanicaldesign and hermetic sealing that numerous other embodiments of theinvention may be envisioned without departing from the spirit and thescope of the invention.

What is claimed is:
 1. A junction for forming a hermetic seal about anoptical waveguide, said junction comprising: a tube for providing fluidcommunication between a first orifice and a second orifice, a capdisposed for covering the first orifice, said cap for reducing a flow ofmolten metal through the first orifice when the optical waveguide isdisposed within the cap, said tube being sufficiently wide to supportthe optical waveguide disposed between the first orifice and the secondorifice while simultaneously permitting molten metal at a predeterminedtemperature to flow within the tube, such that, in use, the opticalwaveguide is disposed within the tube, molten metal is provided withinthe tube and flows to the cap and the molten metal solidifies, therebyforming a hermetic seal between the optical waveguide and the tube.
 2. Ajunction according to claim 1, wherein the cap has a supporting featurefor receiving the optical waveguide, and when supporting the opticalwaveguide, the cap positions the optical waveguide proximate a center ofthe first orifice.
 3. A junction according to claim 2, wherein theoptical waveguide is a ribbon of optical waveguides.
 4. A junctionaccording to claim 2, wherein the tube is straight and has a consistentcross sectional area.
 5. A junction according to claim 4, wherein thecap is for supporting the optical waveguide with strength sufficient forholding the waveguide in tension such that the waveguide is heldsubstantially in the center of the tube, in a state of tension whilemolten metal solidifies within the tube.
 6. A junction according toclaim 1, wherein the cap is held in position using an adhesive.
 7. Ajunction according to claim 2, wherein the opening of the cap is sizedto substantially prevent the flow of molten metal therethrough when theoptical waveguide is disposed therein.
 8. A junction according to claim3, wherein the tube is sized to accommodate the waveguides and permitthe flow of molten thereabout.
 9. A junction according to claim 1,wherein the cap is resilient to a temperature at which the molten metalremains molten.
 10. A junction according to claim 1, comprising apackage for containing an optical component, said tube being integraltherewith.
 11. A junction according to claim 10, wherein the firstorifice is proximate an interior of the package.
 12. A junctionaccording to claim 1, wherein when the molten metal solidifies it bondsto the cap.
 13. A junction for forming a hermetic seal about an opticalwaveguide, said junction comprising: a tube for providing fluidcommunication between a first orifice and a second orifice, said firstorifice for being covered by a cap, said cap for reducing a flow ofmolten metal through the first orifice, said tube being sufficientlywide to support the optical waveguide disposed between the first orificeand the second orifice while simultaneously permitting molten metal at apredetermined temperature to flow within the tube; such that, in use,the optical waveguide is disposed within the tube, a cap is disposedproximate the first orifice, molten metal is provided within the tubeand flows to the cap and the molten metal solidifies, thereby forming ahermetic seal between the optical waveguide and the tube.
 14. A junctionaccording to claim 22, wherein the tube is sufficiently wide to supporta ribbon of optical fibres.
 15. A junction according to claim 14,wherein the tube is sized to accommodate the waveguides and permit theflow of molten thereabout.
 16. A junction according to claim 22, whereinthe tube is straight and has a consistent cross sectional area.
 17. Ajunction according to claim 13, comprising a package for containing anoptical component, said tube being integral therewith.
 18. A junctionaccording to claim 17, wherein the first orifice is proximate aninterior of the package.
 19. A method of forming an optical waveguidehermetic seal comprising the steps of: disposing an optical waveguidethrough a cap; abutting the cap to a first orifice of a tube; providingmolten metal to the tube such that the molten metal provided in achannel of the tube flows toward the first orifice of the tube; and,allowing the molten metal to solidify, thereby forming a hermetic sealbetween the tube and the optical waveguide.
 20. A method of forming anoptical waveguide hermetic seal according to claim 19, comprising thestep of: biasing the molten metal to flow towards the first orifice ofthe tube.
 21. A method of forming an optical waveguide hermetic sealaccording to claim 19, wherein the step of abutting the cap to the firstorifice of a tube comprises the step of using an adhesive for bondingthe cap to the first orifice of the tube.
 22. A method of forming anoptical waveguide hermetic seal according to claim 19, wherein theoptical waveguide is one of a waveguide array and ribbon of opticalfibres supporting at least four optical waveguides.
 23. A method offorming an optical waveguide hermetic seal according to claim 22,wherein the one of a waveguide array and ribbon of optical fibres isstripped to expose a cladding layer of each optical waveguide.
 24. Amethod of forming an optical waveguide hermetic seal according to claim22, wherein the fibres in the one of a waveguide array and ribbon ofoptical fibres are spaced within the tube to provide molten metal flowthereabout and therebetween proximate the cladding layer of each opticalwaveguide.
 25. A method of forming an optical fibre hermetic sealaccording to claim 24, comprising the step of securing the one of awaveguide array and ribbon of optical fibres to a jig prior to the stepof allowing the molten metal to solidify, wherein the jig biases eachoptical waveguide of the one of a waveguide array and ribbon of opticalfibres to position the each of the optical waveguides to facilitate aflow of molten metal thereabout and therebetween.
 26. A method offorming an optical fibre hermetic seal according to claim 24, comprisingthe step of securing the one of a waveguide array and ribbon of opticalfibres to a jig prior to the step of allowing the molten metal tosolidify, wherein the jig biases each optical waveguide of the one of awaveguide array and ribbon of optical fibres to a center of the tube.27. A method of forming an optical fibre hermetic seal according toclaim 19, wherein the step of providing molten metal to the tubecomprises the step of providing molten metal through a second orifice ofthe tube.
 28. A method according to claim 27, wherein the molten metalis a fusable alloy.
 29. A method of forming an optical fibre hermeticseal according to claim 19, wherein the cap has a sleeve feature and thestep of abutting the cap to a first orifice of the tube includesdisposing the sleeve feature within the tube.
 30. A method of forming anoptical fibre hermetic seal according to claim 19, comprising the stepof securing the optical waveguide to ajig prior to the step of allowingthe molten metal to solidify, wherein the jig biases the waveguide to acenter of the tube.
 31. A method of forming an optical fibre hermeticseal according to claim 19, wherein the tube is integral with a packagefor containing an optical component.
 32. A method of forming an opticalfibre hermetic seal according to claim 31, wherein when the step ofabutting the cap to the first orifice is performed the cap is proximatean interior of the package.
 33. A method of forming an optical fibrehermetic seal according to claim 19, wherein the opening of the cap issized to substantially prevent the flow of molten metal therethroughwhen optical waveguides extend therethrough.
 34. A method of forming anoptical fibre hermetic seal according to claim 19, wherein during thestep of allowing the molten metal to solidify the molten metal bonds tothe cap.